Multiblock copolymers having thermoplastic endblocks and an elastomeric midblock are well known from the prior art. Such block copolymers include, for example, polystyrene/polyisoprene/polystyrene (SIS) and polystryene/polybutadiene/polystyrene (SBS). Because these polymers exhibit both thermoplastic properties owing to the thermoplastic endblocks, as well as elastomeric properties due to the elastomeric internal block, these polymers have found a wide variety of commercial applications. One such commercial application employs the multiblock polymer in a blend with a tackifying resin for pressure sensitive adhesive (PSA) compositions. The composition is typically applied to a film, label, tape or other substrate to adhere the film, label, tape or substrate to yet another substrate in a laminated structure.
It is known that radiation-induced crosslinking of PSAs can be utilized to enhance cohesive strength, e.g. temperature performance. For example, the use of multifunctional acrylate monomer crosslinking agents is disclosed in U.S. Pat. No. 4,133,731 to St. Clair et al. However, the volatility and toxicity of such monomers present hazards and processing difficulties in the manufacturing process.
It is also known in the art to functionalize polymers to incorporate inherent radiation reactivity. However, such functionalization methods are typically multistep processes involving complex preparation techniques.
A major challenge in the design of radiation cross-linkable multiblock copolymers suitable for PSA applications is balancing cohesive and adhesive strength properties of the PSA after irradiation. Ordinarily, gains achieved in enhancing shear properties (cohesive strength) by introducing radiation-activated crosslinks is offset by sacrifices in peel and tack properties (adhesive strength). In St. Clair '731, for example, acrylate crosslinking agents are stated to partition between non-elastomeric endblocks and elastomeric midblocks in SIS and SBS triblock copolymers. Solubility factors favor preferential crosslinking by the acrylate monomers in the polystyrene blocks, however, reactivity considerations still favor crosslinking in the rubbery midblocks.
Further efforts involve the use of multifunctional acrylate monomers to crosslink triblock copolymers such as styrene/ethylene-1-butene/styrene (SEBS) block copolymers and block copolymers with a polydiene midblock. The elastomeric blocks are hydrogenated to reduce the occurrence of crosslinking in the rubbery matrix, as disclosed in U.S. Pat. Nos. 4,151,051 and 4,152,231 to St. Clair et al.
A further technique disclosed in U.S. Pat. No. 4,566,464 to St. Clair et al. involves the incorporation of unsaturation into the polystyrene blocks and the use of multifunctional acrylate monomer coupling agents. This is said to enable efficient crosslinking within the polystyrene domains, but the rubbery midblocks still contain relatively high levels of residual unsaturation so that crosslinking in this region has adverse effects on tack properties.
The radiation response of poly(2-phenylbutadiene) is disclosed in Yamaoka et al., "Primary Processes in the Radiation-Induced Crosslinking of Poly(2-Phenylbutadiene)", Polymer Journal, Vol. 19, No. 5, pp. 667-672 (1987). The polymer is stated to contain 88 weight percent of 1,4-units, 6 weight percent of 1,2-units and 6 percent of 3,4-units. The crosslinking is said to occur under gamma-radiation with a G.sub.crosslink (the number of crosslinks per 100 eV radiation energy absorbed) of 7.2. This high value was theorized to be due to benzyl cation and allyl radical intermediates, both of which lead to crosslinking.